Method of fabricating a part made out of ta6zr4de titanium alloy

ABSTRACT

A method of fabricating a part made out of TA6Zr4DE titanium alloy including forging a blank in the alpha/beta domain to form a preform, hot die-stamping the preform to form a rough part in the beta domain of the titanium alloy, and heat treatment. During the die-stamping, the rough part is subjected throughout to local deformation c greater than or equal to 1.2, the die-stamping being terminated by immediate cooling at an initial cooling rate faster than 85° C./min. The method can be utilized to make a rotary part of a turbomachine.

The invention relates to a thermomechanical method of fabricating a part that is made of TA6Zr4DE titanium alloy, and to a part resulting from the method.

The invention applies particularly, but not exclusively, to rotary parts of turbomachines, such as disks, trunions, and impellers, and in particular it relates to high pressure compressor disks.

At present, in the technique used by the Applicant, high pressure compressor disks are obtained by forging, comprising a step of forging a blank in the alpha/beta domain and a step of hot die-stamping in the beta domain of the titanium alloy. The die-stamping is performed at about 1030° C.

This step of die-stamping in a press is followed by a heat treatment cycle comprising a step of solution treatment in the alpha/beta domain of the alloy at a temperature of 970° C., corresponding to the beta transus temperature minus 30° C., for one hour. This solution treatment step is followed by a step of quenching in oil or in a water-polymer mixture.

Thereafter, annealing treatment is performed at 595° C. for eight hours followed by cooling in air.

Without taking particular conditions into account when performing that fabrication method, an alloy is obtained that presents zones of coarse microstructure that are not favorable to good strength of the titanium alloy, in particular during oligo cycle fatigue testing with an imposed stress maintained for a certain dwell time in comparison with the same type of fatigue testing without a dwell time, in particular for a range of utilization temperatures extending from −50° C. to +200° C. The shortening in lifetime observed during such fatigue testing as a result of introducing a dwell time during which the maximum load is maintained leads to a phenomenon that is referred to as the dwell effect. More precisely, it comprises creep at a relatively low temperature (lower than 200° C.) which, coupled with oligo cycle fatigue, leads to internal damage in the material that may go as far as premature collapse of the part.

In particular, it is preferable to use an alloy known as “6242” that includes about 6% aluminum, 2% tin, 4% zirconium, and 2% molybdenum. More precisely, this is known as the TA6Zr4DE alloy in metallurgical nomenclature.

The type of structure that is conducive to the dwell effect phenomenon is shown in FIG. 1: non-tangled needles all presenting a common orientation are located one either side of a grain boundary 10. In this configuration, which may be referred to as a “feather” structure, the needles are parallel to one another.

On the contrary, when the alpha phase needles are thoroughly tangled, i.e. when they are not grouped together in packets of mutually parallel needles but are arranged and distributed with orientations that are very different (see FIG. 1: zone 20, or all of FIG. 2), then a structure is obtained that needs to be encouraged since it is not conducive to the dwell effect phenomenon.

Thus, the application in the field of aviation, and in particular for a high pressure compressor disk is very sensitive to this dwell effect phenomenon because, during stages of takeoff and landing, engines are subjected to operating conditions in the range of temperatures and stresses that correspond to this phenomenon. This phenomenon can lead to premature starts of fatigue cracks, and possibly even to a part breaking.

This dwell effect phenomenon is very well identified by the manufactures of turbomachines and has been the subject of numerous studies; furthermore, it applies to all temperature-stabilized titanium alloys: titanium alloys in the beta, alpha/beta, near alpha, and alpha classes.

An object of the present invention is to provide a method of fabricating a thermomechanical part made of a TA6Zr4DE titanium alloy that can be performed industrially and that makes it possible to overcome the drawbacks of the prior art, and in particular that provides a possibility of limiting the extent of the dwell effect phenomenon.

An object of the present invention is to improve the thermomechanical fabrication method so as to obtain parts for which lifetime relating to the dwell effect phenomenon is increased, in spite of the cyclical stresses to which the parts are subjected at low temperature.

To this end, the present invention relates to a method of fabricating a thermomechanical part made of TA6Zr4DE titanium alloy, the method comprising a step of forging a blank in the alpha/beta domain to form a preform, a step of hot die-stamping the preform in order to form a rough part in the beta domain of the titanium alloy, and heat treatment, the method being characterized in that during the die-stamping step, the rough part is subjected throughout to local deformation s greater than or equal to 1.2, this die-stamping step terminating with immediate cooling at an initial cooling rate faster than 85 degrees Celsius per minute (C/min), and preferably faster than 100° C./min.

The idea on which the present invention is based corresponds to the fact that it has been observed that zones of parallel needles or “colonies” that are conducive to the dwell effect phenomenon exist within the material. It has been found that such colonies are made up of relatively coarse elongate needles of the primary alpha phase that touch one another. Such colonies may present lengths that can be as long as several millimeters, with thickness lying in the range 0.1 millimeters (mm) to 1.5 mm.

When the material is under stress, such colonies constitute locations where large concentrations of dislocations occur, such that once they become active, and without requiring any particular thermal effect, sliding can be generated between the needles, which can lead to breakages.

The present invention seeks to provide a fabrication method that makes it possible to limit grain size and to limit structures of the “colony” type, in particular by seeking to obtain a structure of the “tangled” type, so as to minimize the dwell effect, with this being done by reducing the extent over which dislocations can move freely, so as to minimize accumulation of dislocations and minimize the risk of the part breaking.

That is why, in characteristic manner of the present invention, not only is some minimum level of deformation imparted to the part in order to obtain a microstructure at the end of the die-stamping step that is fine, but it is also ensured that this fine microstructure is conserved by taking the rough part that results from the die-stamping step, and cooling it immediately and sufficiently fast.

By way of example, the cooling terminating the die-stamping is performed by quenching in water, in particular in water at a temperature that does not exceed 60° C.

Advantageously, in this fabrication method in accordance with the invention, said heat treatment includes solution heat treatment in the alpha/beta domain of the alloy immediately followed by cooling at a cooling rate faster than 100° C./min throughout the part.

Preferably, the cooling terminating the solution heat treatment is performed by a step of quenching the part at a cooling rate faster than 150° C./min, and in particular a rate lying in the range 200° C./min to 450° C./min.

Advantageously, the cooling terminating the solution heat treatment is performed by quenching in oil or in a water/polymer mixture.

Thus, because of this fast cooling, the state of the microstructure is frozen in the situation it had at the end of the solution heat treatment step, and any further change to this microstructure is avoided since that might lead to the growth of the needles of the alpha phase colonies that are conducive to the dwell effect phenomenon.

Furthermore, selecting a fast quench helps encourage a martensitic type transformation of the beta phase into an alpha phase (thereby leading to a microstructure that is rather fine), in comparison with the seeding/growth type phenomenon (which leads to a microstructure that is rather coarse).

Also preferably, at the end of the fabrication method in accordance with the invention, the method further includes the following step:

-   -   after the quenching step terminating the solution heat         treatment, performing an annealing step at a temperature of         about 595° C. for a duration of about 8 hours (h), with         subsequent cooling in air.

Advantageously, the fabrication method of the invention also includes, between the die-stamping step (followed by cooling in water) and the solution heat treatment step, a step of machining, and in particular of pre-machining, seeking to diminish the massivity of the part. Other machining operations will follow to rectify the dimensions of the part and reach its final shape.

After the quenching step, the cooling rate should preferably be faster than 350° C./min, if the pre-machining step is added.

In this way, it is possible to reduce the volumes of material that need to be treated during the heat treatment, thereby enabling the part as a whole to be cooled more quickly.

The inventors have found that this method of fabrication that enables the structure to be made finer does not have the consequence of affecting the thermomechanical properties of the material.

The present invention also provides a thermomechanical part made out of a TA6Zr4DE titanium alloy using the fabrication method as described above.

The thermomechanical part made of titanium preferably forms a rotary part of a turbomachine, and in particular a compressor disk, specifically a disk for a high pressure compressor.

Finally, the present invention also relates to a turbomachine fitted with a thermomechanical part complying with any of the definitions given above.

Other advantages and characteristics of the invention appear on reading the following description made by way of example and with reference to the accompanying drawings, in which:

FIG. 1, described above, shows the microstructure obtained with the conventional fabrication method of the prior art;

FIG. 2, described above, shows the microstructure of the type obtained with the fabrication method of the present invention;

FIG. 3 shows the steps of the fabrication methods of the prior art and of the invention; and

FIG. 4 shows the lifetime results of a fatigue test (“trapezoid” cycles with dwell time) at ambient temperature for a part obtained by the fabrication method of the prior art and for a part obtained by the fabrication method in accordance with the invention, with this being done over two zones of the part of different massivity (zones referenced 3 and 5).

With reference to FIG. 3, it is recalled what constitutes the conventional heat treatment of the prior art as used in particular by the Applicant company for high pressure compressor disks that are made of TA6Zr4DE or “6242” titanium alloy.

Initially, a blank or billet of material is forged in the alpha/beta domain, e.g. at 950° C., followed by cooling in air in order to form a preform.

The preform is then subjected to a step of hot die-stamping in the beta domain of the titanium alloy at a temperature of 1030° C., corresponding to the beta transus temperature plus 30° C., followed by cooling in water after forging, thereby obtaining a rough part (also known as a “blank forging”) for forming a disk.

This die-stamping step is followed by heat treatment comprising a solution heat treatment step in the alpha/beta domain of the alloy at a temperature of 970° C., corresponding to the beta transus temperature minus 30° C., for one hour.

This solution heat treatment step is followed by a step of quenching in oil or in a water-polymer mixture (minimum initial cooling rate of about 200° C. and then lying in the range 200° C./min to 450° C./min.

Thereafter, annealing heat treatment is performed at 595° C. for eight hours with cooling in air.

A material is obtained that presents the microstructure visible in FIG. 1, presenting in certain locations colonies that are made up of mutually parallel alpha phase needles situated on either side of a grain boundary. These needles present a section of elongate shape that can be seen in the figure and they often extend over several hundreds of micrometers.

In FIG. 2, the visible microstructure corresponds to that of a titanium alloy identical to the alloy of FIG. 1, and that has been subjected to the above-described fabrication method, with the exception of the following difference:

-   -   during the die-stamping step, the blank is subjected throughout         to local deformation s greater than or equal to 1.2.         Advantageously, this minimum value for local deformation is 1.5,         preferably greater than 1.7, or even greater than 1.9, with most         points exceeding 2.

Under such circumstances, the colonies of parallel needles are less numerous and they are smaller in size.

Most of the needles are tangled and, furthermore, they are of different sizes. As can be seen in FIG. 2, all of the needles are of smaller size in section, their length remaining less than 100 micrometers (μm) and generally lying in the range about 20 μm to 50 μm.

Consequently, it can be expected that the absence of long needles in parallel alignment will prevent the dwell effect phenomenon by preventing dislocations accumulating that might otherwise lead to risks of breakage. The reduction in the size of the needles is accompanied by a reduction in their volume and a reduction in the areas of contact between needles, thereby braking the aptitude for movement of defects such as dislocations or voids, and as a result they travel over shorter distances and they have fewer possibilities for accumulating.

In the present invention, the term local deformation ε is used to mean the equivalent generalized deformation in the Von Mises sense as calculated by the Forge 2005 simulation software. The equation used for calculation is as follows:

$ɛ_{equilvalent}^{plastic}\sqrt{{\left( \frac{2}{3} \right)\lbrack ɛ\rbrack}^{pl}{\text{:}\mspace{14mu}\lbrack ɛ\rbrack}^{pl}}$

where [ε]^(p1) corresponds to the plastic deformation tensor.

In order to be sure that the minimum value for local deformation has been obtained throughout at the end of the die-stamping step, computer-aided design (CAD) means are used to perform a simulation.

In particular, the material that results from the fabrication method as a whole presents thermomechanical characteristics, and in particular properties for withstanding oligo cycle fatigue under all imposed deformations, that are not any worse than those of the materials resulting from the prior art fabrication method.

A test of ability to withstand oligo cycle fatigue with imposed stress has been undertaken using a signal of trapezoidal shape (1 second (s) without stress, 40 s with stress, 1 s without stress) using a maximum stress of 772 megapascals (MPa) at ambient temperature for a high pressure compressor disk.

The results in terms of number of cycles before breakage for tests carried out in a zone 3 (corresponding to a bore) and in a zone 5 (corresponding to the web) of the disks, and visible in FIG. 4, are summarized in Table 1 below:

TABLE 1 Zone 3 Mean Zone 5 Mean Standard 88278 91754 30003 28000 range- 95235 25997 Prior art Invention 150903 139443 105213 120821 127903 (+52%) 136430 (+331%)

Thus, it can be seen that there is an increase in lifetime and thus an increase in ability to withstand the dwell effect phenomenon running from a factor of 1.5 (in zone 3) to a factor of 4 (in zone 5), which is very significant.

Among the other mechanical tests performed by way of comparison and that have demonstrated strength that is at least as great for a part obtained by the fabrication method of the invention as for a part obtained from a standard range, mention may be made of traction tests (at 20° C. and at 450° C.) and creep-lengthening tests at 500° C.

It has also been found that lifetime is increased by a factor of 3 for a part obtained by the fabrication method of the invention compared with a part obtained from a standard range, in terms of a vibratory fatigue test with imposed stress at ambient temperature, at a frequency of 80 hertz (Hz). 

1-12. (canceled)
 13. A thermomechanical method of fabricating a part made of TA6Zr4DE titanium alloy, the method comprising: forging a blank in an alpha/beta domain to form a preform; hot die-stamping the preform to form a rough part in the beta domain of the titanium alloy; and heat treatment, wherein during the hot die-stamping, the rough part is subjected throughout to local deformation E greater than or equal to 1.2, the hot die-stamping terminating with immediate cooling at an initial cooling rate faster than 85° C./min.
 14. A fabrication method according to claim 13, wherein the heat treatment includes solution heat treatment in the alpha/beta domain of the alloy immediately followed by cooling at a rate faster than 100° C./min.
 15. A fabrication method according to claim 13, wherein the cooling that terminates the die-stamping is performed by quenching in water.
 16. A fabrication method according to claim 14, wherein the cooling terminating the solution heat treatment is performed by quenching the part at an initial cooling rate faster than 150° C./min.
 17. A fabrication method according to claim 16, wherein the cooling terminating the solution heat treatment is performed by quenching in oil or in a water/polymer mixture.
 18. A fabrication method according to claim 16, wherein the rate of cooling during the quenching terminating the solution heat treatment is in a range of 200° C./min to 450° C./min.
 19. A fabrication method according to claim 13, further comprising: after the quenching terminating the solution heat treatment, performing an annealing at a temperature of about 595° C. for a duration of about 8 h, with subsequent cooling in air.
 20. A fabrication method according to claim 13, further comprising, between the die-stamping and the solution heat treatment, a machining seeking to reduce massivity of the part.
 21. A thermomechanical part made of a TA6Zr4DE titanium alloy using the fabrication method according to claim
 13. 22. A thermomechanical part according to claim 21, forming a rotary part of a turbomachine.
 23. A thermomechanical part according to claim 21, forming a high pressure compressor disk.
 24. A turbomachine comprising a thermomechanical part according to claim
 21. 